1. Field of the Invention
This invention relates generally to an anode inlet unit for a fuel cell system and, more particularly, to an anode inlet unit for fuel cell systems including a small fuel cell stack for a small vehicle, where the anode inlet unit employs three injectors and two purge valves.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
Flow control pressure regulators are typically employed in fuel cell systems at various locations to provide a desired gas flow rate. For example, flow control pressure regulators are typically employed at the anode inlet of the fuel cell stack to provide a desired pressure reduction and hydrogen gas flow rate for hydrogen gas from a hydrogen pressure storage tank, or other hydrogen storage device.
Solenoid-controlled valves or injectors usually provide pressure regulation and flow control in a fuel cell system. An injector is a 2/2-way valve that switches between a fully opened and a fully closed position at a particular frequency and duty cycle. The frequency determines the time of each switching cycle of the injector, and the duty cycle determines how long the injector is open and closed per cycle, where the ratio of time between the open and closed position of the injector is its duty cycle. An injector can be operated at a constant frequency. However, for low duty cycles, it may be desirable to decrease the frequency because low duty cycles can be adjusted more precisely at low frequencies to increase the injector's turn-down ratio, where the turn-down ration is the ratio of the pressure an the inlet of the valve to the pressure at the outlet of the valve.
Proportional valves are typically not the most desirable for the anode inlet to a fuel cell system because they have a hysteresis that affects fuel cell system operation by making pressure regulation and flow rate more difficult. Also, proportional valves have a low turn-down ratio (1:10). Injectors are usually more desirable because they typically have a large turn-down ratio (1:20) and no hysteresis. In an injector, the flow rate is proportional to the duty cycle of the injector. The opening frequency of the injector can be selected to provide the desired turn-down ratio, where low frequencies are typically required for small flow rates and high turn-down ratios.
Typically, the various flow control valves and injectors for the anode side of a fuel cell stack are part of an anode inlet unit (AIU) that controls the hydrogen gas flow to the anode side of the fuel cell stack. It is typically desirable to make the anode inlet unit as small, lightweight and inexpensive as possible, especially for small vehicles that may employ short stacks. Therefore, it is desirable to reduce the number of valves, pipes, hoses, brackets, fittings, etc. in the anode inlet unit.